1. Field of the Invention
This invention relates to a wet-process gas treatment method and a wet-process gas treatment apparatus by and with which acid gases are treated with an alkaline treating fluid. A preferred embodiment of the present invention relates to a treatment method using a multi-column structure having a plurality of packed columns connected in series. Also, the present invention is preferably applied to a method, and an apparatus, for treating deposits having formed when acid gases generated after the gas cleaning of polysilanes.
2. Related Background Art
Conventionally known as systems of wet-process scrubbers that can deal with a large air flow are a spray column, a packed column, a perforated-plate column and so forth. In these systems, waste gas is introduced from a column""s lower part and a flow of waste gas is formed. Meanwhile, the treating fluid is made to fall from a column""s upper part to form its flow, so that the flow of the waste gas and the flow of the treating fluid counter each other. Namely, in these systems, a treating fluid sprayed in a shower from the column""s upper part and the waste gas are subjected to gas-liquid contact, or the waste gas is passed through a treating fluid layer on a perforated plate so as to be dispersed, to make them undergo gas-liquid contact (see xe2x80x9cSemiconductor Plant Environment Cleaningxe2x80x9d, compiled by Kazumasa Ono, published by Science Forum K.K., published on Dec. 10, 1980).
The packed column has an internal structure as described below.
The column has a gas inlet at its lower part, and a grating (a wire mesh) is provided at a small upper part thereof. This grating frames a space in which the waste gas stays, and at the same time supports packings placed thereon. Then, the packings are layered on the grating, and a spray nozzle from which the treating fluid is sprayed is installed above the upper end of the packings. A column""s upper-part outlet is further connected to a suction fan through a pipe. In some cases, a pressure fan is installed at the waste-gas inlet.
A description of how to design this packed column is provided in xe2x80x9cTheory and Calculation of Chemical Machinesxe2x80x9d (compiled by Saburo Kamei, Sangyo Tosho) Chapter 6, Gas Absorption, Chapter 7, Absorption Problems, pp.177-202. In this publication, an equation is provided according to which, when the gas has a higher mass velocity, the height of the packed column is decreased to increase the inner diameter of the column to lessen any pressure loss. Also, as an absorption problem, a method is disclosed by which the height of a packed column filled with packings and the column""s internal diameter are calculated where the recovery (percentage) of waste gas and the type of packings are fixed.
U.S. Pat. No. Re. 35,234 discloses a case of a multi-column system in which a large number of packed columns of such a type are connected in series. Here, hydrogen sulfide in air is absorbed in a treating fluid to bring it to a desired concentration.
As a use of a wet-process scrubber for acid gases, an apparatus for treating waste gases generated when polysilanes are deposited as by-products in a CVD system is available. In this use, as a countermeasure for such deposition, the above publication discloses use of rotary nozzles as a substitute for usual spray nozzles. Also, in the following publications, techniques are disclosed such that deposits are mechanically scraped out, that a cleaning fluid is made to flow in a spiral form to dissolve deposits and that, in order to prevent blockage of an exhaust duct between a waste-gas treatment apparatus and a production apparatus, a nozzle for spraying a solvent is provided in the interior of the duct.
First, Japanese Patent Application Laid-Open No. 9-186093 discloses that a means for scraping out deposits left in a pipe and a means for exchanging a chemical fluid automatically is provided in order to prevent any blockage due to deposits having formed by the reaction of chloride waste gas with water (Case 1).
Second, Japanese Patent Application Laid-Open No. 4-131121 (Japanese Patent Publication No. 6-77669) discloses that an inlet for introducing untreated waste gases, including halogen-containing silicon compound gases and acid gases, such as hydrogen fluoride gas, or chlorine gas, is provided on a side portion of a jet scrubber, which sprays an alkali cleaning fluid in the form of a high-speed jet from its ceiling, and the cleaning fluid is made to flow down in a spiral flow to this waste gas inlet to prevent blockage due to dust and highly viscous deposits (Case 2).
Third, Japanese Patent Application Laid-Open No. 2000-334409 discloses a means for preventing blockage of the above exhaust duct. In the case disclosed in this publication, the acid-alkali reaction takes place in the exhaust duct to which the waste gases coming from a plurality of production apparatus are gathered, to form reaction products (salts). In this publication, it is shown that a solvent is sprayed from a large number of nozzles and the reaction products are dissolved with the solvent so as to prevent the blockage (Case 3).
As countermeasures for blockage in the conventional wet-process scrubbers, available are, as stated above, scraping-out, chemical-fluid exchange, washing-away, drying prevention, spray nozzle rotating and so forth. However, there are no techniques which can fundamentally prevent deposition of by-products.
Especially where a large quantity of the acid gases to be treated are discharged, a large quantity of gel-like deposits may form in the vicinity of spray nozzles, on the back of spray nozzles, at perforated-plate ends and in the pipes downstream to the nozzles and perforated plates with respect to the flow of acid gas (hereinafter simply xe2x80x9cthe downstream sidexe2x80x9d). The interiors of the pipes on the downstream side tend to become blocked. Once the pipes are blocked, the apparatus must be stopped to disassemble the pipes to clean their interiors. Here, untreated acid gases may diffuse to cause air pollution and lead to an environmental problem. Also, since the acid gases are dangerous to human bodies, those who perform disassembly and cleaning must work wearing chemical protective suits and supplied-air respirators, under local evacuation. Namely, it is necessary to make sufficient considerations for safety.
Where the spray nozzles are rotated to prevent deposits from forming, the deposits may form at non-spray portions of the nozzles, so that the interiors of connecting pipes on the downstream side cannot avoid becoming blocked.
The occurrence of the above problems can be decreased if treating columns having a very large volume are used. This, however, results in high cost of the apparatus and also results in large area for the place where the apparatus is installed.
An object of the present invention is to prevent any pipe blockage due to deposits of silicon compounds or the like, such as SiO2, in a wet-process scrubber having the function to remove harm of acid gases such as hydrogen fluoride gas and chlorine gas. Namely, the present invention aims at simultaneous control of xe2x80x9charm removalxe2x80x9d of hydrogen fluoride gas and chlorine gas and xe2x80x9cdepositionxe2x80x9d. Hereinafter, in the present invention, xe2x80x9charm removalxe2x80x9d and xe2x80x9cgas-liquid contactxe2x80x9d, which are attributable to the treating fluid, are interchangeably used. Also, xe2x80x9ctreating columnxe2x80x9d and xe2x80x9ctreating chamberxe2x80x9d are interchangeably used.
Another object of the present invention is to provide a wet-process gas treatment method and a wet-process gas treatment apparatus, which enable continuous long-time operation and promise safety, preventing the above pipe blockage due to by-product deposits and preventing such deposits from forming on the back of spray nozzles.
To achieve the above objects, the present invention provides a wet-process gas treatment method, which is a method of treating an acid gas by means of at least two treating columns connected in series, wherein
ratio of concentration Cel of acid gas at an outlet to concentration Cil of acid gas at an inlet, R1=Cel/Cil, of a first treating column into which the acid gas is first introduced is higher than ratio of concentration Cen of acid gas at an outlet to concentration Cin of acid gas at an inlet, Rn=Cen/Cin, of at least one of n-th treating columns (nxe2x89xa72).
In the above wet-process gas treatment method, the ratio R1 may preferably be higher than all Rn""s of the n-th treating columns (nxe2x89xa72). Also, the ratio R1 may preferably be 0.4 or more.
The present invention also provides a wet-process gas treatment method, which is a method of treating an acid gas by means of at least two treating columns connected in series, wherein
a mist flow, which is generated from an alkaline treating fluid sprayed from a spray nozzle installed inside a k-th treating column (k is a natural number) and flows on to a k-th+1 treating column, is kept alkaline in any space in pipes, which interconnect the treating columns.
The present invention still also provides a wet-process gas treatment method, which is a method of treating an acid gas by means of at least two treating columns connected in series, wherein
flow rate of a spray, or concentration, of an alkaline treating fluid is so controlled that, with respect to the alkaline treating fluid sprayed into a first treating column, its pH value measured in a treating fluid tank provided at a lower part of the first treating column is 9 or more.
In these treatment methods, the treatment carried out in each treating column may preferably be the same.
The present invention further provides a wet-process gas treatment apparatus, which is an apparatus having at least two treating columns connected in series through connecting pipes, and treating an acid gas with an alkaline treating fluid, wherein
a quantity of packings packed in a first treating column is smaller than a quantity of packings packed in any other treating column.
In this treatment apparatus, the quantity of packings packed in the first treating column may preferably be smaller than any of the quantity of packings packed in n-th treating columns (nxe2x89xa72) (i.e., among quantities of packings packed in all treating columns, the quantity of packings packed in the first treating column is the smallest).
The present invention further provides a wet-process gas treatment apparatus, which is an apparatus having at least two treating columns connected in series through connecting pipes, and treating an acid gas with an alkaline treating fluid, wherein
among the connecting pipes, at least a connecting pipe between a first treating column and a second treating column has spray nozzles to spray into the interior of the connecting pipe in a number of the number of bends plus 1 or more, and the spray nozzles are each so disposed that the alkaline treating fluid is directly sprayed thereon from other nozzle.
In the present invention described above, features may appropriately be combined as long as they are not contradictory to one another. Also, any treatment method making use of the above apparatus is also embraced in the present invention.
In the present invention described above, it is preferable to have at least three treating columns connected in series, in view of an improvement in throughput capacity.